In my U.S. Pat. No. 4,327,795 of May 4, 1982, there is disclosed a window casement designed to be attached to a window frame outside the window sash which is operative on the one hand to serve as a storm window and on the other hand to provide supplemental heat during the winter and cooling during the summer. The casement is hingedly supported at its top to the outer side of the window frame and is provided with means for moving it from a position parallel to the window to a position displaced angularly away from the window frame. The casement defines a closed chamber, except for narrow openings at the top and bottom which provide communication with the interior of the building when the casement is disposed parallel to the window frame and the window is open. There are horizontally hung slats within the casement chamber provided on one side with heat absorbent surfaces and on the other side with heat reflective surfaces. These slats are hung so that they may be rotated about their transverse axes to on the one hand dispose the absorbent surfaces to light entering the casement and on the other hand to dispose the heat reflective surfaces to the light entering the casement. The casement of the instant invention is designed to be a permanent installation in place of the conventional window casement, rather than to be hingedly attached thereto as shown in the aforesaid patent and, optionally, to provide horizontal slats and/or a pull down curtain as shown in the aforesaid patent or vertically hung slats and/or a vertically hung draw drape which can be moved transversely into a storage chamber at one side of the window casement. The vertically hung slats and vertically hung draw drape, as with the horizontally hung slats and pull down curtain shown in the aforesaid patent, have on one side heat-absorbent surfaces and on the other side heat-reflective surfaces.